1. Field of the Invention
The present invention relates generally to optical disk drive tracking and focus servo systems. In particular, the present invention is a method for gain compensating tracking and focus servo systems of a magneto-optic disk drive.
2. Description of the Prior Art
Magneto-optic data recording technology combines the erasability features of magnetic data storage systems with the high data storage capacity of optical systems. A 5.25 inch magneto-optic disk can hold up to 600M bytes of information, 1000 or more times the amount of information that a similarly sized magnetic floppy diskette can store. Magneto-optic disks are also transportable and can be transferred between drives. Since the reading, writing and erasing operations are performed with light beams rather than magnetic heads, they have long life, high reliability, and are relatively immune to physical wear.
The principles of magneto-optic technology are well known. Information is digitally stored at bit positions on a magneto-optic disk. The orientation of the magnetic field at each bit position can be switched between a first or digital one state in which its north pole is oriented upward, and a second or digital zero state in which the magnetic field is reversed and the north pole oriented downward. The orientation of the magnetic field at each bit position is selected by subjecting the bit position to a magnetic field of the appropriate polarity, and heating the bit position of the disk. The magnetic orientation of the bit position is "frozen" when the disk cools and returns to room temperature.
The magnetic fields of all bit positions in an unwritten disk will generally be oriented north poles down to represent digital zeros. When writing information, the bit positions will be subjected to a write magnetic bias field and heated by a high intensity laser beam. The orientation of the magnetic fields at the written bit positions will reverse to north poles up. Bit positions are erased by subjecting them to an erase bias field of the opposite polarity, and again heating the bit position. The magnetic field orientation at the erased bit positions will then reverse and switch to north poles down.
Data is read from the optical disk using a low-power or read intensity laser beam. Because of the magneto-optic phenomenon known as the Kerr Effect, the polarization of a laser beam impinged upon the bit positions will be rotated as a function of the magnetic orientation of the bits. The polarization of laser beam portions reflected from bit positions on the optical disk is detected by opto-electronic detector circuitry. Signals from the detector circuitry are then processed to determine whether the bit position is representative of a digital one or zero.
Bit positions are aligned adjacent one another in an elongated servo track on the optical disk. The optical disk can include a single servo track which is spirally positioned on the disk, or a plurality of concentrically positioned servo tracks. The laser beam used to read, write and erase data at the bit positions is focused onto the disk by an objective lens. Optical disk drives of this type typically include a focus servo system for driving the objective lens about a focus axis, to keep the laser beam in focus the disk. A tracking servo system is used to drive the objective lens along a tracking axis perpendicular to the servo tracks, and to maintain the laser beam centered over a desired servo track.
Tracking and focus servo systems for optical disk drives are generally known and illustrated, for example, in the Silvy et al. U.S. Pat. No. 4,700,056. After the laser beam has been modulated by the individual bit positions, it is reflected from the optical disk and impinged upon an optical detector. Circuitry coupled to the optical detector produces both tracking and focus error signals. The focus error signal is generally sinusoidally shaped and has a magnitude and polarity which represent the distance and direction, respectively, from which the objective lens is displaced from proper focus. Similarly, the tracking error signal is a sinusoidal signal having a magnitude and polarity representative of the distance and direction by which the laser beam is offset from the center of a desired servo track.
The focus and tracking error signals are processed by the servo systems to generate focus and tracking drive signals. The focus and tracking drive signals are applied to respective actuators or motors, and cause the objective lens to be driven to a position which minimizes the focus and tracking errors. Typical processing steps performed on the focus and tracking error signals include the addition of an offset value to compensate for electrical, optical and/or mechanical offsets in the drive, and the multiplication of the error signal by a gain factor to control servo system bandwidth. The error signal is also compensated by lag-lead and/or lead-lag filters for proper frequency characteristics before being applied to the actuator as a drive signal.
Known optical disk drives of the type described above typically use discrete circuit elements for processing the focus and tracking error signals to produce the respective focus and tracking drive signals. The gain factor is adjusted and set by means of a potentiometer during initial setup or calibration of the drive, following its assembly. This procedure is performed by a technician who applies an externally generated test signal having predetermined frequency characteristics to the servo system, and who monitors or records the transfer function or response of the servo system at the frequency of the test signal. From observing the monitored response, and knowing the desired servo system gain at the frequency of the test signal, the technician can adjust the potentiometer to set the gain factor and bring the servo system bandwidth to within required specifications.
From the foregoing description it is evident that known techniques for adjusting servo system gain factors are relatively time consuming and will add to the overall expense of the drive. Since the required gain factor is dependent upon the optical characteristics of the disk, and in particular, variations in track groove geometries, the disk drive is typically used only with a particular type of disk for which it was calibrated. Furthermore, the servo system bandwidth will change as the characteristics of mechanical, electrical, and optical components of the drive vary with age. A service call would generally be necessary to readjust the gain factor.
It is evident that there is a continuing need for improved disk drive servo system gain compensation systems. In particular, what is needed is an adaptable servo system gain compensation system which can accommodate changing characteristics of the drive's mechanical, electrical and optical components, and varying media optical characteristics.